1. Field of the Invention
The present invention relates to sticky polymers and more particularly to a method for preventing agglomeration of sticky polymers in fluidized bed systems.
2. Description of the Prior Art
The introduction of high activity Ziegler-Natta catalyst systems has lead to the development of new polymerization processes based on gas phase reactors such as disclosed in U.S. Pat. No. 4,482,687 issued Nov. 13, 1984. These processes offer many advantages over bulk monomer slurry processes or solvent processes. They are more economical and inherently safer in that they eliminate the need to handle and recover large quantities of solvent while advantageously providing low pressure process operation.
The versatility of the gas phase fluid bed reactor has contributed to its rapid acceptance. Alpha-olefins polymers produced in this type of reactor cover a wide range of density, molecular weight distribution and melt indexes. In fact new and better products have been synthesized in gas phase reactors because of the flexibility and adaptability of the gas phase reactor to a large spectrum of operating conditions.
The term "sticky polymer" is defined as a polymer, which, although particulate at temperatures below the sticking or softening temperature, agglomerates at temperatures above the sticking or softening temperature. The term "sticking temperature", which, in the context of this specification, concerns the sticking temperature of particles of polymer in a fluidized bed, is defined as the temperature at which fluidization ceases due to excessive agglomeration of particles in the bed. The agglomeration may be spontaneous or occur on short periods of settling.
A polymer may be inherently sticky due to its chemical or mechanical properties or pass through a sticky phase during the production cycle. Sticky polymers are also referred to as non-free flowing polymers because of their tendency to compact into agglomerates of much larger size than the original particles. Polymers of this type show acceptable fluidity in a gas phase fluidized bed reactor; however, once motion ceases, the additional mechanical force provided by the fluidizing gas passing through the distributor plate is insufficient to break up the agglomerates which form and the bed will not refluidize. These polymers are classified as those, which have a minimum bin opening for free flow at zero storage time of two feet and a minimum bin opening for free flow at storage times of greater than five minutes of 4 to 8 feet or more.
Sticky polymers can also be defined by their bulk flow properties. This is called the Flow Function. On a scale of zero to infinity, the Flow Function of free flowing materials such as dry sand is infinite. The Flow Function of free flowing polymers is about 4 to 10, while the Flow Function of non-free flowing or sticky polymers is about 1 to 3.
Although many variables influence the degree of stickiness of the polymer resin, it is predominantly governed by the temperature and the crystallinity of the resin. Higher temperatures of the resin increase its stickiness while less crystalline products such as very low density polyethylene (VLDPE), ethylene/propylene monomer (EPM), ethylene/propylene diene monomer (EPDM) and essentially amorphous or elastomeric polypropylene usually display a larger tendency to agglomerate to form larger particles.
Low pressure polymerizations in a gas phase reaction of olefin polymers using transition metal catalysis are generally performed at temperatures below 120.degree. C. Where the higher levels of comonomers are used and crystallinity levels are reduced below 30%, the melting or softening temperature of these olefin polymers can be close to the polymerization temperatures which are used. Under such conditions in either a fluidized or stirred gas-solid phase reactor, stickiness of the olefin polymer particles or granules becomes a problem. Ethylene copolymers using propylene, butene-1, and higher alpha comonomers are prone to stickiness problems when their crystallinity is below 30% or densities less than 910 kg/m.sup.3. The stickiness problem becomes even more critical with copolymers of ethylene and propylene, and their diene terpolymers (EPM and EPDM, or EPRs) having a crystalline content less than 10%.
The stickiness problem in a fluidized bed or a gas-phase reactor can be reduced by the introduction of selected, fine-particle size, inorganic materials which act as a fluidization or flow aid. Certain grades of carbon black, clay and silica have been shown to be useful for this purpose (see copending application Ser. No. 07/413,704 filed Sep. 28, 1989 and which is assigned to a common assignee). Further the treatment of alpha-olefin polymers with low levels of polydimethylsiloxane (PDMS) has been proposed as a means of diminishing adhesion of the polymer particles to themselves and to the reactor walls (see for example, European Patent application no. 0-254-234 filed Jul. 17, 1987 and assigned to Mitsubishi Chemical Industries Limited and U.S. Pat. No. 4,675,368 issued Jun. 23, 1987).
However, the surface treatment of granular EPR with PDMS in a concentration range of 0.01 to 5.0% by weight has not been found to be effective in preventing sticking under fluidized bed conditions. Furthermore, there are processing difficulties (e.g., uniform coating of dispersion without liquid binding) in directly treating the EPRs either in-situ or in post reaction in a continuous reaction with such PDMS because of their relative high viscosities.
The present invention relates to improvements in reducing stickiness of polymer particles not only in the gas-solid phase reactor but also in the post-reactor handling units. These improvements are obtained when selected, fine-particle size, inorganic materials, such as carbon black, clay, talc, or silica, are surface treated with PDMS, polymethylhydrogen siloxane (PMHS), or alkyl modified (AM)-PDMS (low alkyl content) having viscosities in the 10 to 100,000 cSt range all of which are hereinafter described as "polydimethylsiloxane" (PDMS) and such treated fluidization aids are used in a gas-solid phase reactor.